四舍五入时，DecimalFormat是否不考虑十进制值？

Question

Am trying to format the amount.But the decimal values not being rounded in correct manner.

public class Testing {    
    private java.text.DecimalFormat dispAmt;    
    public Testing() {                
    }    
    public static void main(String args[]){   
       Testing testing=new Testing();    
       testing.dispAmt = new java.text.DecimalFormat("##,##,##0.00");   
       // Line #8
       System.out.println(testing.dispAmt.format(1974.545));     
       System.out.println(testing.dispAmt.format(1974.535));   
    }   
 }    

OutPut:
=========
1,974.54    
1,974.54

In above prog what's wrong with 8th line. why it is not rounding as "1,974.55"?? where am doing mistake!! pls suggest..

Answer 1

By default DecimalFormat uses rounding mode of HALF_EVEN which

behaves as for RoundingMode.HALF_UP if the digit to the left of the discarded fraction is odd; behaves as for RoundingMode.HALF_DOWN if it's even.

In this case the second last least significant digit of 4 of 1974.545 is even so the value is rounded down. Conversely, the value of 3 is odd so rounding up occurred there in the subsequent format statement.

Try using RoundingMode.HALF_UP .

testing.dispAmt.setRoundingMode(RoundingMode.HALF_UP);

Answer 2

DecimalFormat#format is not rounding! it is just cutting off the values you don't display by default.

DecimalFormat uses half-even rounding (see ROUND_HALF_EVEN) for formatting.

http://docs.oracle.com/javase/6/docs/api/java/text/DecimalFormat.html

you have to set the RoundingMode manually

dispAmt.setRoundingMode(RoundingMode.HALF_UP);

Answer 3

You need to set the RoundingMode for the formatter by setting it using the setRoundingMode() method, as by default, it does not round the value.

dispAmt.setRoundingMode(RoundingMode.CEILING); // Ex 1
dispAmt.setRoundingMode(RoundingMode.HALF_UP); // Ex 2 but I guess you need this in your case

There are many RoundingMode s available. Do check the docs for that.

Answer 4

DecimalFormat is a concrete subclass of NumberFormat that formats decimal numbers. And this doesn't round up the given numbers. INFO This just use the pattern you are given to display the number.

DecimalFormat provides rounding modes defined in RoundingMode for formatting. By default, it uses RoundingMode.HALF_EVEN.